Electromagnetic deflection yokes have been used for many years, and their technology is well-defined. The electromagnetic field through which an electron beam in a cathode ray tube travels on its journey from the electron gun in the tube neck to the phosphor target at the viewing screen determines the sweep or deflection pattern it experiences. Normally simultaneous horizontal and vertical deflection is effected to produce a raster of illuminated phosphor. With monochrome picture tubes, the shape and position of the pattern is not as critical as it is with color picture tubes because the latter actually have three electron beams producing corresponding patterns which must be converged throughout.
For many years, a delta or triangular arrangement of electron guns has been used in conjunction with a shadow mask positioned close to a screen consisting of a mosaic of different colored light-emitting phosphor elements. As is well known, the shadow mask apertures shield or "shadow" each color deposit from beams from two of the three electron guns, allowing it to be impacted only by the beam from its associated electron gun. Thus (ideally) the "red" beam only strikes the red phosphor deposits, the "blue" beam only the blue deposits and the "green" beam only the green deposits.
Tri-color tubes have historically required numerous external mechanisms for converging the electron beams at the shadow mask throughout the tube viewing area. A principal reason is that the electron beam sources are not located at the origin of the sphere defined by the radius of curvature of the picture tube target screen which results in the electron beams travelling farther as the deflection angle increases.
A vast simplification in external convergence hardware has been made possible by the development of the so-called in-line type electron gun. Indeed, with proper design of the deflection yoke, in-line gun picture tubes can be made that require no external convergence apparatus. However, there are constraints placed on yoke positioning. In delta gun picture tubes positioning of the yoke along the tube neck was required. In "self-converged" in-line type tubes the yoke axis must also be movable with respect to the tube axis to achieve convergence. These movements take the form of vertical and horizontal tilt adjustments, which physically offset the yoke from the tube axis until the electron beams coincide with the phosphor deposits.
There are three basic yokes types; the saddle, the toroid, and the hybrid. In a saddle type, both the horizontal and vertical deflection coils are formed to the approximate contour of the picture tube funnel-neck area and arranged within a liner surrounded by a magnetic core. In the toroid form the windings are wound around the core in the direction of its central axis. The hybrid yoke generally has a saddle horizontal deflection coil and a toroidal vertical coil. All yokes employ an insulation liner of some type for at least securing the yoke assembly to the tube neck. In toroid yokes of the so-called precision type, the positions of the individual coil turns are maintained within close tolerances, and their liners are quite rigid often with peripheral serrations to securely position the individual winding turns.
Most in-line picture tubes use self-converged deflection yokes (either the toroid or hybrid type), and the necessary yoke tilt adjustments are made and held by exerting appropriate forces on the peripheral areas of the yoke assembly, generally the edges of the funnel end of the liner.
One prior art structure comprises a full toroid yoke which the manufacturer physically cements in position on the picture tube. Such a structure is exemplified by U.S. Pat. No. 3,786,185 issued Jan. 15, 1974 and includes an annular platform separately cemented to the funnel of the picture tube. This provides a surface for tilting the yoke assembly (with suitable means) until the desired operational yoke-tube relationship is obtained. The entire assembly is then cemented into position. The yoke is not thereafter adjustable and both the yoke assembly and the picture tube are replaceable as a unit. In this system, there is no positive clamp support for the yoke assembly on the tube neck. The platform provides the sole support for the yoke assembly.
U.S. Pat. No. 4,006,301, issued Feb. 1, 1977, shows a hybrid yoke and also includes a platform or ring which is cemented to the funnel of the picture tube and provides a surface with respect to which the yoke assembly may be tilted for proper orientation about the tube axis. Here the cemented platform supports the yoke assembly in conjunction with a clamp on the rear of the yoke assembly housing which anchors it to the tube neck. A plurality of wedges are used to tilt the liner of the yoke assembly with respect to the tube axis by forcing wedges between the platform and the liner front. The entire assembly is constructed of plastic with the wedges being slidably retained in place by a ratchet and locking mechanism. The wedges may be released for readjustment of yoke tilt by operation of release tabs. This system works well, is reasonably economical, but suffers from its two-piece construction and physical size. Its two-piece construction precludes complete fabrication of the yoke assembly at a single manufacturing facility because one of the pieces must be attached to the picture tube during final assembly of the television receiver. Its size makes the unit very difficult, if not impossible, to use with small tubes because of the limited rear access space available in such receivers.
Other adjustment devices which are self-contained, i.e.; do not have separate parts or pieces that must be cemented to the tube, are known in the art. In particular, one manufacturer includes a heavy, rigid plastic liner having three screw-bearing attachments which are positioned about, and locked into place on, the periphery of the funnel portion of the liner. The screws are made of plastic and include ends for contacting the picture tube funnel and slotted heads for screwdriver adjustment thereof. In conjunction with the yoke housing clamp, which secures the rear of the assembly to the neck of the picture tube, the screws provide tilt adjustments for the yoke assembly with respect to the tube axis. The screws are maintained in the desired position by cementing their threaded portions. While the cement bond may be broken for readjustment, this is not easily accomplished and the glue set-up time can adversely affect the receiver assembly line.
Another structure utilizes a much lighter weight liner with a plurality of molded holes for reception of small screws. A heavy ring with six adjustment screw support members (only three of which are used) is attached to the liner by a number of these small screws. The tilt adjustment screws are similar to the ones mentioned above and are also cemented to lock them in position when the desired yoke tilt is achieved.
While these latter two self-contained structures facilitate rapid and positive tilt adjustments, they are very expensive and lack a convenient method for subsequent readjustment of the yoke. In both yoke structures, the tilt screws are positioned to orthogonally intersect the funnel surface. Consequently, the screws are set at about a 120.degree. angle to the front surface of the liner. This makes it difficult to reach the slotted ends for adjustment when the tube is mounted to its chassis. These and other problems of the prior art structures are overcome in a facile and economical manner with the system of the invention.